The present invention relates to an electrodeless discharge lamp for emitting light by an electromagnetic field generated by an induction coil disposed in a re-entrant portion or cavity of a bulb.
In recent years, from the viewpoint of protecting global environment, discharge lamps having higher efficiency and longer life in comparison with incandescent lamps have been used widely. Further, research and commercialization of electrodeless discharge lamps having remarkably longer life in comparison with conventional discharge lamps having electrodes in a discharge space are being carried out earnestly. Not having the electrodes in the discharge space case the limited life of the conventional discharge lamps, the electrodeless discharge lamps have significantly extended life. For this reason, diffusion of the electrodeless discharge lamps in the future is expected.
In this kind of electrodeless discharge lamp, a discharge plasma is generated in the discharge space by a high-frequency electromagnetic field generated by an induction coil arranged in a cavity of a bulb, resulting in that the lamp emits light. The induction coil is formed of a winding wire wound around a magnetic core made of a magnetic material and has the shape of a solenoid of a finite length. Generally, a ferrite material is used widely for the magnetic core. The lamp is driven by a high-frequency power having a frequency of several tens of kHz to several tens of MHz and supplied to the winding wire.
Japanese Patent Application Laid-Open 60-72155 discloses a structure of a typical induction coil shown in FIG. 14. An electrodeless low-pressure mercury vapor discharge lamp shown in FIG. 14 has a discharge vessel or bulb 101 made of glass and filled with mercury and krypton. An induction coil 103 and a magnetic core 104 are accommodated in a tubular cavity 102 of the bulb 101. A cross-sectional area of the magnetic core 104 is in the range of 20 to 60 mm2. The induction coil 103 is formed of a winding wire 105 directly wound 10 to 15 turns around the magnetic core 104.
Japanese Patent Application Laid-Open 10-92391 discloses two structures respectively shown in FIGS. 15 and 16. One structure shown in FIG. 15 is a type of that a winding wire of an induction coil is directly wound around a magnetic core. The other structure shown in FIG. 16 is a type of that a bobbin is provided between a magnetic core and an induction coil. In the structure shown in FIG. 15, a pair of fingers 202 is integrated with a base 201 for supporting a bulb (not shown). These fingers 202 extend so as to pass through a cylindrical magnetic core 204 around which an induction coil 203 is wound. The fingers 202 have protruding portions 202a for supporting the magnetic core 204 at one ends opposite to the base 201. The magnetic core 204 is supported by a spring washer 205 so as not to wobble. In the structure shown in FIG. 16, an induction coil 303 is wound around a coil bobbin 302 integrated with a base 301 for supporting a bulb. A magnetic core 304 is held in a groove formed in the inner circumferential face of the coil bobbin 302.
However, the above-mentioned conventional electrodeless discharge lamps have problems described below.
In the case of magnetic core made of a material having relatively low electric conductivity, such as Ni—Zn ferrite, the probability of dielectric breakdown is low even when no particular consideration is given to the insulation between the magnetic core and the winding wire. However, in the case that a driving frequency of a drive circuit is between 50 kHz and 1 MHz, there is a possibility that a material having relatively high electric conductivity, such as Mn—Zn ferrite, Cu—Zn ferrite, silicon steel plate, or permalloy, is used for the magnetic core. When such a material having relatively high electric conductivity is used for the magnetic core, it is essential to secure high insulation reliability between the magnetic core and the winding wire.
However, induction coils of which the winding wires are directly wound around the magnetic core as shown if FIGS. 14 and 15 make it considerably difficult to secure insulation reliability. Specifically, at winding start and winding end portions of the winding wire, the winding wire is apt to be bent acutely or sharply. Even if the winding wire is covered with insulating coating, the acute or sharp bending of the winding wire is apt to cause not only uneven flatness or uneven thickness in the insulating coating but also damages of the insulating coating. As a result, the dielectric breakdown occurs easily between the magnetic core and the winding wire or between adjacent portions of winding wire.
Further, a number of turns of the induction coil tends to become larger as the driving frequency of the lamp is lower. While strength of an induced electric field in the bulb required for generating and maintaining the discharge plasma hardly changes with variation of the driving voltage, strength of an induced electric field due to magnetic fluxes generated by the induction coil is proposal to the driving frequency. For this reason, as the driving frequency is lower, it is necessary to increase the number of turns of the induction coil in order to increase the magnetic fluxes. More specifically, in the case that the driving frequency is low, it is necessary to increase the number of turns by reducing an interval between adjacent portions of the winding wire (winding pitch) or by winding the winding wire in multi-layer. Therefore, in case that the drive voltage is a relatively low value of 1 MHz or less, it is essential to secure insulation between the winding wires.
Wherein the coil bobbin 302 is provided between the magnetic core 304 and the induction coil 303 as shown in FIG. 16, it is possible to prevent the dielectric breakdown between the winding wire of the induction coil 303 and the magnetic core 304. However, an outside diameter of the induction coil 303 becomes larger due to a wall thickness of the coil bobbin 302. This results in that a size of the cavity of the bulb needs to be increased. The whole size of the bulb is limited by sizes of a lighting apparatuses being generally used. For this reason, as the cavity become larger, the discharge space inside the bulb eventually becomes smaller, causing increase of a diffusion loss of the discharge plasma which occurs a possibility that the luminous efficiency of the lamp is adversely affected.